The need for effective herbicides needs no special emphasis. The control of weeds and undesirable vegetation is of great economic importance since weed competition inhibits the production of foliage, fruit or seed and agricultural crops. The presence of weeds can reduce harvesting efficiency and the quality of the harvested crop. Thus, suppression of undesirable weed growth is very advantageous.
Herbicides are generally used to control or eradicate weed pests on a variety of crops. The particular emphasis of the instant invention is the suppression of undesirable weeds at the locus of rice plants. Herbicides are useful because they increase rice yields and reduce harvest costs.
Some herbicides may injure rice plants at application rates necessary to control weed growth. To be effective, an herbicide must cause at most minimal damage to the rice plant while maximizing injury to weed species which infest the locus of the rice plant. To preserve the beneficial aspects of herbicides which injure plants beyond acceptable levels, many herbicide safeners have been prepared. These safeners reduce or eliminate damage to the rice plant without substantially impairing the damaging effect of the herbicide on weed species.
U.S. Pat. No. 4,964,893 ("Brannigan et al.") incorporated herein by reference, discloses certain benzhydryl compounds as effective safeners for a variety of herbicides. These herbicides include thiocarbamates, triazines, diphenylethers, benzoic acid derivatives, phenyl ureas and acetamides. Brannigan et al. also disclose that the combination of one or more of the foregoing herbicides with one or more of the safeners disclosed therein is effective for the control of weeds with concomitant low crop injury. These crops include corn, grain sorghum, and cereals such as wheat, rice, barley, oats a rye as well as soybeans and cotton.
However, as is outlined in Crop Safeners for Herbicides, ed. by K. K. Hatzios and R. E. Hoagland, Academic Press, San Diego, 1989,
"[a]ll the presently available safeners exhibit a high degree of botanical and chemical specificity and protect only certain crops against injury from selected groups of herbicides." Id. at 66. PA1 a) an herbicidally effective amount of acetochlor; and PA1 b) a non-phytotoxic antidotally effective amount of a compound according to formula (I), ##STR6## wherein each of R.sup.1 and R.sup.2 independently represents one or more substituents selected from hydrogen, (C.sub.1 -C.sub.6)alkyl, (C.sub.1 -C.sub.6)alkoxy, (C.sub.1 -C.sub.6)alkylamino, halo, halo(C.sub.1 -C.sub.6)alkyl, (C.sub.1 -C.sub.6)alkoxy-(C.sub.1 -C.sub.6)alkyl and nitro; PA1 a) an herbicidally effective amount of acetochlor; and PA1 b) a non-phytotoxic antidotally effective amount of a compound according to formula (I), ##STR9## wherein each of R.sup.1 and R.sup.2 independently represents one or more substituents selected from hydrogen, (C.sub.1 -C.sub.6)alkyl, (C.sub.1 -C.sub.6)alkoxy, (C.sub.1 -C.sub.6)alkylamino, halo, halo(C.sub.1 -C.sub.6)alkyl, (C.sub.1 -C.sub.6)alkoxy-(C.sub.1 -C.sub.6)alkyl and nitro;
Thus, it appears that Brannigan et al. is overly broad in its determination of what combinations of herbicide and safeners will be effective in view of what is known in the art.
In general, safening activity is determined empirically by observing the complex interaction of several factors. Some of these factors include the interaction of the herbicide compound, the type of weed to be controlled, the crop to be protected, and the safening compound itself.
The herbicide used in the present invention is 2-chloro-N-(ethoxymethyl)-6'-ethyl-o-acetotoluidide. It has the common name acetochlor and has the following chemical structural formula: ##STR3##
A particularly effective class of safening compounds with use of acetochlor are of the formula ##STR4## wherein each of R.sup.1 and R.sup.2 independently represents one or more substituents selected from hydrogen, alkyl, alkoxy, alkylamino, halo, haloalkyl and nitro; wherein A is selected from ##STR5## wherein each of R.sup.3 and R.sup.4 is independently selected from hydrogen, alkyl, haloalkyl, cyanoalkyl, alkenyl, alkynyl, phenyl, nitrophenyl, benzyl, halobenzyl, pyridinylalkyl and alkylammonium; and n is 0 or 1.
The safening of acetochlor for some crop species is known in the art. In fact, Brannigan et al. disclose acetochlor safening with some of the safeners above in sorghum, corn and soybean. In addition, the same patent discloses a well known herbicide, butachlor, 2-chloro-2',6'-diethyl-N-(butoxymethyl) acetanilide, use on rice with an appropriate safener. However, the use of acetochlor on rice plants is not disclosed.
The compounds disclosed herein have been found to be especially useful for safening rice against injury caused by acetochlor. As discussed more fully below, rice may be grown or cultured using a variety of methods. The following is a description of different rice culturing methods:
In the postflood postemergence (transplanted) method, rice is grown to the 2-4 leaf stage away from the field. The field is flooded and tilled (puddled) until a blend of mud is achieved. The rice plants are then transplanted into this mud. Herbicide application typically takes place before or after flooding.
In the postflood postemergence (water seeded) method, rice is soaked for 24 or more hours, then is sown to the surface of a shallow flooded field. Herbicide application is typically after weed germination.
In the preflood postemergence direct seeded (broadcast or drilled) method, rice is broadcasted or planted with a planter under the soil surface. The field may be flushed (watered) to promote rice growth. The field is flooded 1 week or more after this planting as the plants germinate. Herbicide application takes place typically before this flood, but after emergence of the rice plants.
In the preflood postemergence (South East Asia style) method, rice is soaked for 24 or more hours. The field is puddled to the right consistency and drained. The pre-germinated seeds are then broadcast to the surface of the soil. Flooding takes place as the rice develops. Herbicide application normally takes place before the flooding, but after the emergence of the rice plants.
Finally, in the preemergence or delayed preemergence method, seeds are planted usually with a planter of sorts. Herbicide application is made before emergence of the rice or weeds.
In China, acetochlor has been applied for weed control in rice in a postflood, postemergence application at very low concentration levels of active ingredient. However, this use has not produced good results because no safener is employed and thus the rice plants suffer phytotoxic effects.
Although the use of acetochlor on other crop species is known, it was thought to be too injurious to rice at the concentration levels necessary for effective weed suppression. Surprisingly, the inventors have found that acetochlor may be used on rice plants at effective levels when a safener is employed. High levels of weed control are possible with acetochlor and with the appropriate safener, rice injury is reduced to an acceptable level.